Every precaution had been taken except one—flexibility

From the Guns of August by Barbara Tuchman.

The plan of campaign was as rigid and complete as the blueprint for a battleship. Heeding Clausewitz’s warning that military plans which leave no room for the unexpected can lead to disaster, the Germans with infinite care had attempted to provide for every contingency. Their staff officers, trained at maneuvers and at war-college desks to supply the correct solution for any given set of circumstances, were expected to cope with the unexpected. Against that elusive, that mocking and perilous quantity, every precaution had been taken except one—flexibility.